This invention relates to a method for preparing spherular silica gel particles, especially to a method for preparing spherular silica gel particles suitable as a carrier of a catalyst for initiating the polymerizing of olefin.
Spherular silica gel particles have been widely used as carriers for a catalyst used to initiate the polymerization of olefin. Since the spherular silica gel particles are inactive porous particles, they are desirable carriers of the polymerization-initiating catalyst. .Furthermore, spherular silica gel particles have desirable fluidity for vapor-phase polymerization.
The physical properties, such as surface area, average pore diameter, pore volume, particle strength, and average pore diameter, of the spherular silica gel particles used as carriers should be strictly controlled. These physical properties of the spherular silica gel particles should be generally kept within the following ranges:
surface area--50 to 700m.sup.2 /g; PA1 average pore diameter--20 to 1000 .ANG.; PA1 pore volume--0.3 to 2.0 ml/g; PA1 average particle diameter--5 .mu.m to 5 mm; and PA1 particle strength--1 to 4.
However, spherular silica gel particles with physical properties satisfying the above ranges have been difficult and expensive to prepare.
One known method for preparing spherular silica gel particles is disclosed in Japan Published Examined Patent Application No. S54-9588. By the method disclosed therein, colloidal silica is formed into spherular particles with an average particle diameter of 0.5 .mu.m to 50 .mu.m using hydrophilic organic substances as a binding agent. However, this method requires an intricate step where the mixture of prepared silica sol particles and the organic substances are sintered at 500.degree. C. minimum for four hours or longer to remove polymerized organic substances. This method has another problem caused by the fact that the physical properties of the spherular silica gel product are hard to control because the physical properties of the original colloidal silica determine those of the silica gel product.
On the other hand, a method for preparing spherular silica gel particles without using organic substances is proposed in Japan Published Unexamined Patent Application No. S53-65295 In the method described therein, silica sol is spray-dried to obtain spherular silica gel particles. However, irregular silica gel particles such as hollow or indented gel particles result from this method. These silica gel particles have less fluidity and have varying particle strength.
Another known method is proposed in Japan Published Examined Patent Application No. S47-35676. The method described therein comprises the steps of mixing sodium silicate and sulfuric acid at 23.degree. C. to 50.degree. C., preparing a mixture solution of a ph 9.8 to a ph 10.4 to form silica hydrogel slurry, aging the silica hydrogel slurry, decreasing the pH to 3 or below, increasing the pH between 8 and 10.2, aging, and spray-drying. However, since this method requires many intricate steps, it is an industrially disadvantageous method. Further, the particle strength of the resulting silica gel particles is hard to control.
Another known method is proposed in Japan Published Unexamined Patent Application No. S62-275014. The method described therein comprises the steps of reacting sodium silicate solution and mineral acid to prepare silica hydrogel slurry, mixing aqueous ammonia with the silica hydrogel slurry, wet-grinding and spray-drying. However, by this method, the particle strength of the silica gel product is hard to control.